1. Technical Field
Biopharmaceutical process development with recombinant protein producing mammalian cells has realized a tremendous increase in both productivity and product yields in the past years. These achievements can be mainly attributed to the advancements in cell line development, media, and process optimization. Only recently, genome-scale technologies enable a system-level analysis to elucidate the complex biomolecular basis of protein production in mammalian cells promising an increased process understanding and the deduction of knowledge-based approaches for further process optimization.
2. Background
Biopharmaceutical process development faces the challenge to develop high titer processes for the production of clinical-grade material for toxicology studies obeying tight project timelines. Within this time-frame the design of the expression system, the generation and selection of stable high-producer cell clones (mainly CHO cells, hybridomas, BHK, and NS0 cells), and the design of scalable bioprocesses including media optimization and process control need to be addressed to maximize cell specific productivity and product yields. In recent years, we have seen enormous product titer increases in recombinant mammalian cell culture with, for example, product concentrations clearly above 5 g/L for immunoglobulins produced in CHO cells today. Achievements in molecular and cell biology including cell line engineering, in media design, and in process control strategies (e.g. by nutrient feeding) paved the way to this progress.
Though aiming at a more macroscopic bioprocess analysis, e.g. by application of online spectroscopy, knowledge-based continuous process improvements based on comprehensive process data analysis is also brought forward by the FDA as manifested in its Process Analytical Technology (PAT) initiative that motivates continuous and innovative (bio)pharmaceutical process development.
From a process science perspective there exists the challenge that further process improvement will require a more detailed knowledge of the (intra)cellular production system itself, that is an increased understanding of the physiological phenotype as observed in a given cultivation system (e.g. microtiter plate, shake flask, bench or production-scale bioreactors). At this, the main challenge is given by the fact that the flow of biological information and material flux in a cell occurs at many different levels and follows complex control mechanisms. The advancement of large-scale technologies, such as transcriptomics, proteomics or metabolomics, can provide plenty of data on the state of a biological system. However, the genetic and physiological properties that make a certain cell to a high producer cell are complex and not fully understood yet (Seth G, Charaniya S, Wlaschin K F, Hu W S. In pursuit of a super producer-alternative paths to high producing recombinant mammalian cells. Curr Opin Biotechnol 2007; 18:p 557-564).
Essentially, the application to industrial process development requires that the data from these genome-scale technologies can be transformed into information amenable to bioprocess design. Gene expression profiling allows large-scale transcript data generation on well established and robust experimental platforms. However, important physiological information, e.g. on post-translational modifications, enzyme activities or metabolic fluxes, can not be elucidated by this technology. Moreover, industrially relevant mammalian cell lines such as Chinese hamster ovary cells (CHO) still lack genome sequence information.